The present invention relates to mobile terminals for use in analog and digital-based cellular communication systems, and, in particular, to an improved antenna configuration for dual-band operation.
Although experiments have been performed from ancient history forward in the realm of electricity and magnetism, it was not until the early 1900s that the electromagnetic spectrum was harnessed for commercial communication by Guglielmo Marconi and his antennas. As is known to those skilled in the art of communications devices, an antenna is a device for transmitting and/or receiving electromagnetic signals. A transmitting antenna typically includes a feed assembly that induces or illuminates an aperture or reflecting surface to radiate an electromagnetic field. A receiving antenna typically includes an aperture or surface focusing an incident radiation field to a collecting feed, producing an electronic signal proportional to the incident radiation. The amount of power radiated from or received by an antenna is described in terms of gain.
At its simplest, electromagnetic fields or waves originate with time-varying electrical currents. The focus of antenna design thus can be boiled down to producing the right currents when desired. While Marconi used huge antenna arrays with seventy-meter towers, operating at wavelengths of approximately 2000 to 20,000 meters, modem antennas typically correspond to a mathematically ideal antenna known as the half-wave dipole antenna. That is, the antenna""s total length corresponds to a length equal to half the wavelength of the operating frequency.
While referred to as half-wave antennas, the physical dimensions of the antennas may be much shorter than a half-wavelength at an operating frequency. This is effectuated by creating an effective electrical length of the antenna equal to a half-wavelength. This electrical length is dictated by the resistance, inductance and capacitance (collectively the impedance) of the conductors used to form the antenna. The elements of the impedance are functions of the physical dimensions of the conductors used to form the antennas as well as functions of frequency. The resulting impedance is made up of a real part (the radiation resistance) and an imaginary part (the reactance). The reason half-wave dipole antennas are popular is due, in part, to the fact that the imaginary part of the impedance of the antenna disappears when the antenna is approximately a half-wavelength. Such antennas are said to be resonant.
Another important factor in antenna design is the Voltage Standing Wave Ratio (VSWR), which relates to the impedance match of an antenna feed point with the impedance of a feed line or transmission line of a communications device, such as a radiotelephone. To radiate radio frequency (RF) energy with minimum loss, or to pass along received RF energy to a receiver with minimum loss, the impedance of an antenna should be matched to the impedance of the transmission line or feeder.
Since Marconi""s time, the use of antennas in everyday life has exploded. Antennas are now ubiquitous, begin present in radios, telephone, televisions, and many other domestic and commercial devices. Of particular interest are mobile communications terminals. Mobile terminals, and especially mobile telephones and headsets, are becoming increasingly smaller. These terminals require a radiating element or antenna for radio communications. There are presently four frequency stages set aside by the communications authorities as appropriate channels which are commonly used to effectuate mobile radio communications, namely AMPS (824-894 MHz); GSM900 (880-960 MHz); PCS (1850-1900 MHz); and DCS (1710-1880 MHz). A good antenna is designed to operate at least over the entire length of one of the designated frequency ranges. It is preferable to have an antenna which operates over two of the designated channels, such antennas commonly being referred to as dual-band antennas. Many examples exist of single and dual-band antennas.
Conventionally, antennas for such hand held terminals, whether single or dual band, are attached to and extend outwardly from the terminal""s housing. These antennas are typically retractably mounted to the housing so that the antenna is not extending from the housing when the terminal is not in use. With the ever decreasing size of these terminals, the currently used external antennas become more obtrusive and unsightly, and most users find pulling the antenna out of the terminal housing for each operation undesirable. Furthermore, these external antennas are often subject to damage during manufacture, shipment, and use. The external antennas also conflict with various mounting devices, recharging cradles, download mounts, and other cooperating accessories.
Well known in the art as a result of the experiments of Brown and Woodward is the bowtie antenna. In its basic embodiment, the bowtie antenna includes a rectangular dielectric material with a longitudinal axis. Triangular shaped conductors are disposed on opposite sides of the dielectric material and extend from the center of the longitudinal axis outwardly towards the opposing ends of the rectangular shape. The bow tie antenna is a dipole antenna.
Also known in the antenna art is a meander antenna, which is structured somewhat similarly and is likewise a dipole. The meander antenna includes a rectangular dielectric material with a longitudinal axis and a pair of sinuous, relatively narrow conductors disposed on opposite sides of the material which extend from the center of longitudinal axis outwardly towards the opposing ends of the rectangular shape. The sinuous shapes are rectilinear and extend laterally across the rectangular shape. The meanders behave differently at different frequencies. At lower frequencies, such as 800 MHz bands, the electrical length of the radiating elements is typically the longest. At mid-range and high frequencies, such as 1500 and 1900 MHz bands, the electrical length of the radiating elements becomes shorter. At the higher frequencies, the wavelength becomes smaller and this reduces the effect of the meander, because the energy can jump over the oscillations of the meanders.
The meander antenna is also a dual-band antenna. Commonly owned application Ser. No. 09/089,433 describes a multiband combination bowtie-meander-dipole antenna for a cellular telephone, and is incorporated herein by reference.
As phone designs become increasingly smaller, antennas inevitably are brought closer to the ground plane within the phone. As antennas are brought closer to the ground plane, typically the printed circuit board (PCB) of the phone, antennas in general, and the bowtie and meander antennas in particular, begin to lose their effectiveness. It has been discovered that the effective bandwidth of the antenna is narrowed as the antenna is brought closer to the ground plane of the antenna. Also, tuning of the resonance frequencies becomes problematic due to the strays and parasitics caused by the antenna""s close proximity to the ground plane. The conventional approaches of using extra traces and tuning elements may not provide sufficient bandwidth in both bands of operation in many situations. Also, lumped elements such as capacitors and inductors do not adequately eliminate strays and parasitics.
Additionally, the bowtie-meander antenna suffers a further problem not experienced by other antennas as it is brought close to the ground plane. Not only does the bandwidth narrow at the lower frequency, but also the resonance at the high band disappears, thus causing a dual band antenna to change into a single band antenna. In localities where single band operation is acceptable, the loss of a frequency band may not be a large problem, but consumers now expect their radio telephones to operate on a plurality of systems, such operations requiring the use of multiple frequency bands.
Accordingly, there remains a need for a dual-band antenna that will operate effectively in two operating bands even when the antenna is brought in close proximity to the ground plane of the phone.
The present invention provides an internal antenna for mobile terminals that provides performance comparable with externally mounted antennas, even when placed in close proximity to the ground plane. The antenna comprises a dielectric substrate oriented generally perpendicularly to a ground plane and two radiating elements arranged in a dipole configuration. The radiating elements are disposed on opposing surface of the dielectric substrate. The antenna may use the printed circuit board of the mobile terminal as the ground plane. Alternatively, the antenna may have a ground plane oriented perpendicularly to the printed circuit board. Orienting the antenna perpendicular to the ground plane allows the antenna to resonate at two or more different frequencies.
The radiating elements preferably include a bowtie element and a meander element having a plurality of oscillations. The bowtie elements are disposed in a center portion of the substrate. The meander elements extend outward from the bowtie elements toward opposite ends of the substrate. The antenna may be tuned to the desired frequency bands by adding parasitic tuning elements by varying the length, width and shape of the radiating elements, by varying the thickness or dielectric constant of the substrate, by varying the spacing of the antenna from the ground plane or by a combination thereof.
An advantage of the present invention is that it allows the design engineer to match the antenna to a Voltage Standing Wave Ratio (VSWR) of approximately 2:1 in two distinct operating bands (typically the 900 MHz and 1800 MHz bands) even at the band edges. This VSWR allows the antenna to obtain broad bandwidth in both frequency bands of operation and reduces loss of gain due to mismatch of the VSWR. No prior art antennas have been able to obtain these advantages in an antenna in such close proximity to the ground plane.